Such ceramic gas sensors have been known for a long time. These gas sensors are used for detecting toxic exhaust gases, for example, of automobile exhaust gases or burner exhaust gases. The ion-conducting ceramic must be heated for ensuring a proper function and this ceramic can, for example, be zircon oxide (ZrO.sub.2). The heater is embedded in an insulating layer which, in most cases, comprises aluminum oxide (Al.sub.2 O.sub.3). The insulation of the heater is, however, problematic especially at high temperatures because leakage resistances occur from the heater to the sensor electrodes in the order of magnitude of several megaohms. In this case, the leakage current, which flows from the heater, is superposed on the actual measuring current and causes relatively large errors. This is so because in gas sensors of this type, the measuring result lies only at a few microamperes.
Furthermore, in gas sensors, there is a large scattering thereof and a significant deterioration occurs. For this reason, the leakage current cannot be compensated simply by a fixed corrective quantity.
For this reason, it is, for example, known to use an insulating amplifier for measuring the measurement current. With this insulating amplifier, the actual measurement circuit is operated so as to be separated with respect to potential and the measurement quantity is transmitted in an insulated manner for further signal processing. The potential separation can, for example, be achieved via transformers or optoelectronically. In this case, the leakage current cannot flow off and therefore vanishes. As a consequence of this, only the wanted measurement current is measured. The high requirement of expensive and non-integratable components or components which are only integratable with difficulty is disadvantageous.
Furthermore, a method and an arrangement for detecting a fault state of a .lambda.-probe and the measures taken as a consequence of a fault signal are presented in EP 0 403 615 B1. This fault signal is outputted for a detected fault state. In the method, impermissibly large fault causes are diagnosed by a correlation method during operation of the .lambda.-probe. Shunt currents exist only during operation of the heater. For this reason, the probe heater is switched off in this measuring method for detecting a fault state because, in this case, no shunt voltage and therefore no incorrectness of the measuring result is present. The difference between the probe voltage, which is measured for a switched-on heater and a switched-off heater, yields the shunt voltage. This method is relatively complex and requires a rather complex circuit.